1. Field of the Invention
The present invention relates to an electric power steering system for vehicles and, more particularly, to a motor-controlled power steering system for assisting operation of a steering wheel through controlling an electric motor.
2. Description of Related Art
There have been known motor controlled power steering systems and hydraulically controlled power steering systems. Such a power control system actualizes a desired assist characteristic by regulating a controlling quantity for the electric motor or a controlling hydraulic quantity according to a turning torque applied to a steering wheel and/or a steering speed (which is a differential value of turning torque). It is known from, for example, Japanese Unexamined Patent Publication 8-72734 to change the assist characteristic according to a vehicle speed or a yaw rate in addition to a vehicle speed.
In the prior art electric power steering system a motor controlling quantity for the electric motor is determined by multiplying a turning torque applied to a steering wheel, that is detected by a torque sensor such as a torsion bar type torque sensor disposed between a steering wheel and one of front wheels, by a specified assist control gain. The assist control gain is predetermined based on tests conducted on a given vehicle so as to provide a desired assist characteristic.
However, there are possibly variations in assist characteristics of power steering systems. This causes that the feeling of steering varies with the assist characteristic of power steering system. The variation in assist characteristics is due, for example, to variations in inertia and variations in electric motor property and/or variations in friction of parts of reduction gears of steering apparatuses that are disposed between a steering shaft and an electric motor. The variation in friction is predominantly due to manufacturing errors of the parts of the steering apparatus. Specifically, when the part has a magnitude of friction greater than an ordinary magnitude of friction, thrust of the electric motor is spent for the friction even though the electric motor is controlled with a motor controlling quantity that is determined by multiplying the turning torque applied to the steering wheel by the assist control gain. As a result, the electric motor encounters a lack of assist torque for steering, so as to cause a feeling of tottery operation of the steering wheel.
The variation in assist characteristic is further due to assembling errors of, for example, the reduction gear. The reduction gear comprises a worm gear and a worm wheel that have a comparatively large reduction gear ratio. In order for the reduction gear to prevent or significantly reduce backrush and rattling noises, the worm gear is strongly forced against the worm wheel. This causes relatively large variations in friction due to assembling errors because of a high magnitude of friction between the worm gear and the worm wheel forced against each other and has a significant influence on the variations in assist characteristic.
The prior art electric power steering system has the problem of yaw overshooting of front wheels. Since the yaw overshoot cancellation property of wheels (which is referred to the action of wheels such that the front wheels smoothly restore a steering angle of 0 (zero) due to a self-aligning torque without overshooting when letting go a hold on the turned steering wheel) has repercussions on the straight ahead controllability of vehicle, the prior art electric power steering system implements the damping control for braking the electric motor with a controlling quantity of damping that is determined by multiplying a rotational speed of motor by a damping control gain in addition to the assist control. Similarly to the assist control gain, the damping control gain is regulated so that the yaw overshoot cancellation property of wheels is desirable. Therefore, there is the problem that the desired yaw overshoot cancellation property of wheels is not attained. This is due to non-restoration of the steering wheel to the center or neutral position (straight ahead traveling position) or shakes of the steering wheel at the center or neutral position due, for example, to the variations in inertia and/or friction.
In order to eliminate the aforesaid problem, it is valuable to get rid of manufacturing errors of parts of the steering apparatus or to employ high precision parts of the steering apparatus for reducing friction. However, in addition to a change in the magnitude of friction due to expansion or contraction of parts of the steering apparatus that are caused following a change in temperature or a change in magnitude of friction due to aging of parts of the steering apparatus, the assist characteristic and the yaw overshoot cancellation property of wheels possibly change due to a change in gross vehicle weight resulting from a change in loadage. In consequence, it is hard to eliminate the aforesaid problem even when employing high precision parts of the steering apparatus. The utilization of high precision parts causes additional costs of manufacturing the steering apparatus.
An object of the present invention is therefore to provide an electric power steering system for a vehicle that always provides steering apparatuses with a desired assist characteristic and a desired yaw overshoot cancellation property of wheels.
It is another object of the present invention to provide an electric power steering system which provides a steering apparatus with an improved follow-up property of wheels to operation of the steering wheel.
It is a further object of the present invention to provide an electric power steering system which is provided at low costs.
The above object of the present invention is accomplished by an electric power steering system for a vehicle which performs feedback control of a steering apparatus on the basis of a turning torque applied to a steering wheel in addition to assist control for controlling an electric motor so as to assist a turn of the steering wheel.
According to a preferred embodiment of the present invention, the electric power steering system comprises a torque sensor disposed between the steering wheel and front wheels of the vehicle and operative to detect a turning torque applied to the steering wheel, first control means for determining such a first control quantity as to countervail the turning torque when the electric motor is controlled with the first control quantity, second control means for operating a target rate of change in wheel steering angle on the basis of the turning torque and for determining a second control quantity by subtracting an actual rate of change in wheel steering angle from the target rate of change in wheel steering angle, and motor control means for determining a motor control quantity by adding the first control quantity and the second control quantity together and for controlling the electric motor with the motor control quantity.
The torque sensor detects a turning torque applied to the steering wheel when the steering wheel is turned to steer the vehicle. The first control means determines such a first control quantity as to countervail the turning torque when the electric motor is controlled with the first control quantity, in other words determines a first control quantity representative of an assist torque by multiplying a turning torque by a specified control gain. The second control means determines a target rate of change in wheel steering angle on the basis of the turning torque and then determines a second control quantity by subtracting an actual rate of change in wheel steering angle from the target rate of change in wheel steering angle. The operation of the target rate of change in wheel steering angle may be made based on a vehicle model of a mechanical system of an steering apparatus between the torque sensor and a tire on the front wheel or a vehicle model in which factors such as inertia of an electric motor and a knuckle arm and spring component and dumping component of a tire are taken into consideration. The motor control means controls the electric motor with the motor control quantity which is determined by adding the first control quantity and the second control quantity together.
When considering that the control of the electric motor with the first control quantity does not provide a desired rate of change in wheel steering angle due to friction and inertia of the steering apparatus, there is an occurrence of a deviation between an actual rate of change in wheel steering angle and the target rate of change in wheel steering angle determined on the basis of a turning torque applied to the steering wheel that is detected by the torque sensor. Therefore, the control of the electric motor with the second control quantity that is determined on the basis of the deviation causes the electric motor to generate thrust which the electric motor is short of in order to provide the target rate of change in wheel steering angle, so that the vehicle is always steered at a desired rate of change in wheel steering angle. In this instance, because the second control quantity is determined on the basis of a target rate of change in wheel steering angle that is independent from friction and inertia of the steering apparatus, the rate of change in wheel steering angle is always desirable with respect to a turn of the steering angle irrespective of variations in friction and inertia. This clears variations in assist characteristic among steering apparatuses.
That the torque sensor detects a turning torque applied to the steering wheel indicates a response delay of a change in wheel steering angle with respect to driver""s turning operation of the steering wheel. However, the control of the electric motor with the second control quantity causes the electric motor to increase thrust while the torque sensor detects a turning torque applied to the steering wheel. This makes the electric motor to quickly follow up the turning operation of the steering wheel. As a result, a feeling of tottery operation of the steering wheel is eliminated and the follow-up property of wheels is improved.
Furthermore, when letting go a hold on the turned steering wheel, the torque sensor detects a turning torque of 0 (zero), this indicates that the target rate of change in wheel steering angle is determined to be 0 (zero). Accordingly, the electric motor is controlled with the second control quantity so that the front wheels attain a rate of change in wheel steering angle of 0 (zero), thereby preventing the steering wheel from tottery operation and is provided with an improved yaw overshoot cancellation property of wheels, consequently. Whenever the torque sensor detects a turning torque of 0 (zero), the target rate of change in wheel steering angle is 0 (zero). The front wheels, and hence the steering wheel, is always centered with a desire characteristic irrespective of variations in friction and inertia. This eliminates variations in the yaw overshoot cancellation property of wheels among steering apparatuses. On the other hand, in the case where the front wheels change a steering angle due to road surface irregularities, while the torque sensor detects a turning torque of 0 (zero), the front wheels are such that they are unintentionally steered at an uncertain rate of change in wheel steering angle is not 0 (zero). In this event, the second control means controls the electric motor so that the front wheels attain a rate of change in wheel steering angle of 0 (zero), thereby keeping the vehicle in a straight ahead traveling state.
This control is actualized by additionally providing the second control means in the prior art electric power steering system. This does not need extra sensors and the like, so that the steering apparatus can be the same in mechanical structure and operation as those that have been practically used. Therefore, the electric power steering system of the present invention provides desired assist characteristics and a desired yaw overshoot cancellation property of wheels and the follow-up property of wheels, the straight ahead controllability of vehicle, all of which can be realized with complex control beyond the scope of a low cost power steering system.
According to another preferred embodiment of the present invention, a wheel steering angle is used as one of parameters in place of a rate of change in wheel steering angle. Specifically, the electric power steering system comprises a torque sensor disposed between the steering wheel and front wheels of the vehicle and operative to detect a turning torque applied to the steering wheel, first control means for determining such a first control quantity as to countervail the turning torque when the electric motor is controlled with the first control quantity, second control means for operating a target wheel steering angle on the basis of the turning torque and for determining a second control quantity by subtracting an actual wheel steering angle from the target wheel steering angle; and motor control means for determining a motor control quantity by adding the first control quantity and the second control quantity together and for controlling the electric motor with the motor control quantity.
The second control means determines a target wheel steering angle and determines a second control quantity by subtracting an actual wheel steering angle from the target wheel steering angle. The operation of target wheel steering angle may be made based on a vehicle model as previously described. The motor control means controls the electric motor with the motor control quantity which is determined by adding the first control quantity and the second control quantity together.
Even when the control of the electric motor with the first control quantity does not provide a desired wheel steering angle due to friction and inertia of the steering apparatus, the control of the electric motor with the second control quantity, i.e. a deviation between of an actual wheel steering angle from the target wheel steering angle, causes the electric motor so that the front wheels attain the target wheel steering angle. In this instance, because the target wheel steering angle is independent from friction and inertia of the steering apparatus, the wheel steering angle is always desirable with respect to a turn of the steering angle irrespective of variations in friction and inertia. This clears variations in assist characteristic among steering apparatuses.
When there is an occurrence of a response delay of wheel steering angle with respect to driver""s turning operation of the steering wheel, the control of the electric motor with the second control quantity causes the electric motor to increase thrust. Therefore, the electric motor is controlled so as to quickly follow up the turning operation of the steering wheel. This results in improved follow-up property of wheels.
The electric motor is controlled with the second control quantity so as to attain a wheel steering angle of 0 (zero) when the torque sensor detects a turning torque of 0 (zero). Therefore, when letting go a hold on the turned steering wheel, the electric motor is controlled so that the front wheels attain a steering angle of 0 (zero). This realizes quick centering of the front wheels, and, in consequence, the steering wheel is prevented from tottery operation and is provided with a more improved yaw overshoot cancellation property of wheels. The control of the electric motor with the second control quantity always provides a specified yaw overshoot cancellation property of wheels irrespective of magnitude of friction and inertia of the steering apparatus. This eliminates variations in the yaw overshoot cancellation property of wheels among steering apparatuses. On the other hand, in the case where the front wheels change a steering angle due to road surface irregularities, while the torque sensor detects a turning torque of 0 (zero), the front wheels are such that they are unintentionally steered at an uncertain rate of change in wheel steering angle is not 0 (zero). In this event, the second control means controls the electric motor so that the front wheels attain a rate of change in wheel steering angle of 0 (zero), thereby keeping the vehicle in a straight ahead traveling state.
This control is actualized by additionally providing the second control means in the prior art electric power steering system. Therefore, the electric power steering system of the present invention provides desired assist characteristics and desired yaw overshoot cancellation property of wheels and the follow-up property of wheels, the straight ahead controllability of vehicle, all of which can be realized with complex control beyond the scope of a low cost power steering system.
The control that is performed by the electric power steering system of the first embodiment is such as to bring the rate of change in wheel steering angle to 0 (zero) when there is no turning torque that is detected by the torque sensor, which is different from the control performed by the electric power steering system of the second embodiment in which the wheel steering angle is brought into 0 (zero) in the same event. Therefore, the electric power steering system of the first embodiment possibly causes such an action as reducing a self-aligning torque on the front wheels or the steering wheel. On the other hand, the control that is performed by the electric power steering system of the second embodiment is based on the steering displacement of front wheel (a wheel steering angle) possibly be slow in response of steering to a turn of the steering wheel as compared with the electric power steering system of the first embodiment in which the control is performed based on the steering speed of front wheel (a rate of change in wheel steering angle).
In light of the above problems, the electric power steering system according to another preferred embodiment employs a target rate of change in wheel steering angle as a second control quantity and a target wheel steering angle as a third control quantity in order to determine a motor control quantity. Specifically, the electric power steering system comprises a torque sensor disposed between the steering wheel and front wheels of the vehicle and operative to detect a turning torque applied to the steering wheel, first control means for determining such a first control quantity as to countervail the turning torque when the electric motor is controlled with the first control quantity, second control means for operating a target rate of change in wheel steering angle on the basis of the turning torque and for determining a second control quantity by subtracting an actual rate of change in wheel steering angle from the target rate of change in wheel steering angle, third control means for operating a target wheel steering angle on the basis of the turning torque and for determining a third control quantity by subtracting an actual wheel steering angle from the target wheel steering angle; and motor control means for determining a motor control quantity by adding the first control quantity, the second control quantity and the third control quantity together and for controlling the electric motor with the motor control quantity.
Because the electric power steering system has both of the second control means that determines a target rate of change in wheel steering angle on the basis of a turning torque applied to the steering wheel and determines a second control quantity by subtracting an actual rate of change in wheel steering angle from the target rate of change in wheel steering angle and the third control means that determines a target wheel steering angle on the basis of the turning torque applied to the steering wheel and determines a third control quantity by subtracting an actual wheel steering angle from the target wheel steering angle, it always provides a desired assist characteristic and a desired yaw overshoot cancellation property of wheels and enhances the follow-up property of wheels, the yaw overshoot cancellation property of wheels and the straight ahead controllability of vehicle similarly to the electric power steering systems of the first and second embodiments. Further, the electric power steering system has superior responsiveness and yaw overshoot cancellation property of wheels beyond the electric power steering systems of the first and second embodiments.
In order to eliminate the above problems, it is effective to regulate the sensitivity of second control quantity. Specifically, the sensitivity of second control quantity may be changed greater with an increase in vehicle speed, an increase in gross vehicle weight, a decrease in wheel steering angle or a decrease in rate of change in wheel steering angle. Otherwise, the sensitivity of second control quantity may be changed smaller with a decrease in road surface friction coefficient. Further, the second control means implements a high-pass filtering treatment of the second control quantity.
According to still another embodiment of the present invention, the electric power steering system comprises a torque sensor disposed between the steering wheel and front wheels of the vehicle and operative to detect a turning torque applied to the steering wheel, first control means for determining such a first control quantity as to countervail the turning torque when the electric motor is controlled with the first control quantity; second control means for operating a target rate of change in wheel steering angle on the basis of the first control quantity and for determining a second control quantity by subtracting an actual rate of change in wheel steering angle from the target rate of change in wheel steering angle; and motor control means for determining a motor control quantity by adding the first control quantity and the second control quantity and the third control quantity together and for controlling the electric motor by the motor control quantity.
The electric power steering system of this embodiment exhibits the same effects as that of the first embodiment. In addition, although it is necessary for the second control means to determines a target rate of change in wheel steering angle in consideration of a control gain by which a turning torque applied to the steering wheel is multiplied like the determination of first control quantity in the first control means, it is unnecessary to take a control gain into consideration when determining the target rate of change in wheel steering angle on the basis of the first control quantity since the first control quantity has been determined in consideration of the control gain. This makes operation in the second control means simple. In addition, this makes it unnecessary that the second control means stores data of control gains.
According to a further preferred embodiment of the present invention, the electric power steering system comprises a torque sensor disposed between the steering wheel and front wheels of the vehicle and operative to detect a turning torque applied to the steering wheel, first control means for determining such a first control quantity as to countervail the turning torque when the electric motor is controlled with the first control quantity, second control means for operating a target wheel steering angle on the basis of the first control quantity and for determining a second control quantity by subtracting an actual wheel steering angle from the target wheel steering angle, and motor control means for determining a motor control quantity by adding the first control quantity and the second control quantity together and for controlling the electric motor with the motor control quantity.
In addition to the effect attained by the electric power steering system of the second embodiment, the electric power steering system of this embodiment that is structured such as to determine a target wheel steering angle on the basis of the first control quantity can be simple in operation in the second control means like the electric power steering system of the third embodiment and avoids the necessity of using a control gain in the second control means.
In the third and fourth embodiments, it is effective to structure the electric power steering system such that the first control means makes a correction of the first control quantity on the basis of a vehicle speed and a rate of change in wheel steering angle and the second control means makes a correction of the second control quantity on the basis of a vehicle speed. In the case of making a correction of the first control quantity on the basis of a vehicle speed and a rate of change in wheel steering angle and a correction of the second control quantity on the basis of a vehicle speed, if implementing parallel determination of the first and second control quantities, there occurs apprehensions of weakening the correlation between the first and second control quantities. This leads to aggravation of the follow-up property of wheels and the yaw overshoot cancellation property of wheels and causes a deterioration of steering feeling. However, according to the electric power steering system in which the second control means determines a target rate of change in wheel steering angle or a target wheel steering angle on the basis of a second control quantity determined in the second control means enhances the correlation between the first and second control quantities. This is true for the case where the first control means makes a correction of the first control quantity on the basis of a vehicle speed and a rate of change in wheel steering angle and the second control means makes a correction of the second control quantity on the basis of a vehicle speed. This prevents or significantly reduces aggravation of the follow-up property of wheels and the yaw overshoot cancellation property of wheels and improves a steering feeling.
As described above, the electric power steering system of the present invention controls the electric motor with a second control quantity determined on the basis of a rate of change in wheel steering angle or a wheel steering angle that is independent from variations in friction and inertia, so that the steering apparatus is always operated with a desired assist characteristic and a desired yaw overshoot cancellation property of wheels irrespective of magnitude of friction and inertia. This clears variations in performance among steering apparatuses. The control of the electric motor with a second control quantity improves the follow-up property of wheels to turning operation of the steering wheel. In addition, when the torque sensor detects a turning torque of 0 (zero), the control of the electric motor endeavors to make the target rate of change in wheel steering angle or the wheel steering angle 0 (zero), as a result of which, there is provided improvement in the yaw overshoot cancellation property of wheels and the straight ahead controllability of vehicle. Regulating the sensitivity of second control quantity according to a change in vehicle speed, gross vehicle weight, wheel steering angle or wheel steering angle realizes more desirable assist characteristics and yaw overshoot cancellation property of wheels. In addition, the determination of second control quantity on the basis of the target rate of change in wheel steering angle or the wheel steering angle that is determined on the basis of a first control quantity makes the second control means simple in operation and enhances the correlation between the first and second control quantities. As a result, a steering feeling is significantly improved.